Course: The Boston course has it's share of hills including a downhill first 4 miles, 4 rough hills between 16 and 21 miles and another downhill section from 21-24 miles. The net result is a net downhill of 400+ feet (120+ meters). How this effects each persons time will differ depending on conditioning, tactics used and physiological make-up. I agree with Renato that stronger built runners have an advatage (i.e. De Castella ran 2:07 at Boston).
Weather: This includes wind, temperature and humidity. Temperature and humidity on Monday were condusive to good marathoning. The wind was perfect - strong sustained tailwind but not unreasonably hard.
Tactics: Runners often benefit (probably more emotionally than physically) from being able to have someone else lead them at an approriate pace. History has shown us that the fastest races are usualy run with either even or slightly negative splits. While the elite men did not have a rabbit, they did have a runner (Hall) that was willing to lead them at what turned out to be Geoffery and Moses' ideal pace (allowing a slightly negative split performance) through the first 15 miles of the race.
In my humble opinion all of these things played a signifcant role in what we saw in the elite men's race. We saw two men who's physiological make-up and conditioning where well suited for the Boston course. Alot has been made of their very fast 5k split after the top of heartbreak hill. But they where well suited for that, they have good speed (sub 27:00 10k speed as Renato pointed out) and that 5k is a significant down hill, and they where in the second half of a negative split performance (i.e. the first half didn't take it all out of them). This is where they had a huge advantage over a someone like Hall who's 10k fitness is a minute slower than theirs. Hall, no matter how strong, can't produce a 5k that late in the race at that close to his 5k best, while Geoffery and Moses where not nearly as close to theirs.
I think the top 2 men where signifciantly better suited for the Boston course than most runners (as Renato stated). Strong enough and fast enough to take advantage of the 400+ net downhill, and most specifcally the downhill from 21-24 miles.
Next the tailwind was definately a factor, how much is almost impossible to tell. Renato estimates 40 seconds and others 3 minutes. Who's right? I think it depends on the person, their size, weight, if they ran in a pack or out in the open, etc. We can use some math and science and come up with average advantage but to apply an average to any 1 individual may be misleading.
Tactics: As Geoffery said after the race, Hall has to be credited with a superb job as rabbit. He wasn't trying to rabbit them but in essense that is what he did for the top 2 men. Without Hall in the race I think it likely the end result would have been 1-2 minutes slower. I have often said that I think the key to a fast Boston time is to arrive at the top of heart break with enough fuel left in the tank to take advantage of the downhill from 21-24 miles. The top 2 runners did just that.
So where does that leave us? With this wonderful thread where we all debate that which isn't quantifable or knowable. We can come up with averages for each variable but that doesn't tell us how factors affected a specifc individual.
For what it's worth, my opinion is that I don't think Boston is a particularly slow course for certain runners who run certain tactics. It's a net downhill course, so one would think that if you are conditioned and run in such a way, you can take advantage of that or at least negate the effect of the hills. I think Geoffery and Moses did just that. I think the tactics they used (thanks again to Hall) was spot on, a slightly negative split arriving at the top of heart break hill ready to take advantage of the down hill section (21-24). Lastly I think the wind helped significantly, I would estimate in the neighborhood of 1-2 minutes. This puts Geoffery and Moses in the 2:04-2:05 range with no wind, consistent with Renato's expectations and Geoffery's history.
Anyways that is my 2 cents for what it's worth.